Incineration and decomposition of chemicals by plasma have been known for over 50 years. For example, during the 1950's ethylene was produced by the cracking of hydrocarbons with plasma. Subsequently, the technology was practically abandoned by the chemical industry for a few decades.
During the past 15 years the plasma incineration of hazardous materials has once again become popular due to several factors. First of all, recent environmental concerns have raised emission standards for the destruction of hazardous materials. These standards can be followed by conventional incinerators only with great difficulty and at relatively high expense. Furthermore, there exists a wide class of organic wastes, such as halo-organic compounds, whose incineration in the presence of air can result in the production of compounds even more hazardous than the starting materials, such as phosgene, dioxins, furans and other extremely toxic gaseous substances. The treatment of such wastes by conventional pyrolysis is extremely expensive.
Furthermore, the temperature level which can be reached in combustion is limited by the adiabatic flame temperature. Since most conventional incinerators operate at 1200.degree. C., total destruction of waste requires high retention time, post-combustion and fast quenching time, complex gas cleaning, etc. Due to all of the above factors, the use of plasma incinerators has become commercially acceptable and sometimes even the sole treatment method. By using plasma one can achieve temperatures as high as 2000-3000.degree. C. in the treated bed so that the cracking of molecules is done faster and more efficiently. Another important aspect is that there is no need to add an oxidant agent to enable pyrolysis. This fact is important for example, for halo-organic compounds where the presence of oxygen can produce toxic substances.
Conventional plasma incinerators are generally operated in a continuous mode which will be referred to as continuous plasma incineration (CPI). The plasma is produced by a discharge in a carrier gas such as air, oxygen, nitrogen or argon so that the plasma has a very low density. Therefore, although the plasma can reach a high temperature, the effective temperature in the treated bed is in the range of 2000-3000.degree. C. Conventional CPI incinerators are operated at a power level of a few MW.
Plasma can also be generated by pulsed-plasma incineration (PPI) in a pulsed form by confined high pressure discharge, as described for example in A. Loeb and Z. Kaplan, IEEE Transaction on Magnetics, Vol. 25, No. 1, 342 (1989), incorporated by reference. PPI has a basically different mode of operation than that of CPI devices. This method of generation very efficiently couples a large amount of electrically stored energy into the formation of a hot plasma jet. Pulses of a duration of several milliseconds and at a power level of up to 1GW can be-produced.
Furthermore, following the ignition of the discharge in a confined volume, the plasma begins to ablate the surrounding wall material. This has several important consequences. Firstly, the plasma density is greatly increased, usually up to 10.sup.-3 g/cc, due to ablated mass being added to the plasma without the need of any carrier gas. Secondly, the ablated matter cools the plasma down to 1-3 eV, and subsequently increases the device's electrical impedance. If the plasma is allowed to escape from the confined volume through an exit nozzle, steady state plasma production operation conditions can result for a steady voltage/current supply to the discharge.
Furthermore, the high density plasma jet which carries a mass in the order of 100 mg per pulse travels at a velocity in the range of 10-20 km/s. Hence, the jet carries a very high momentum.
In the CPI case, on the other hand, the plasma radiation is. absorbed by a very small amount of the treated matter or by the gases in the reactor due to the low density and the low jet momentum. Thus, the remainder of the treated bed is heated mainly by conduction and convection heat transfer mechanisms. The energy is evenly distributed to all the degrees of freedom in the treated bed. In the PPI mode the dominant mechanism is radiative heat transfer (RHT). This mechanism is generally ineffective. However, in PPI the RHT is increased by several orders of magnitude due to the propagation of the high-velocity jet in the treated bed. This is due to the fact that the surface area exposed to the radiation is substantially increased by the Rayleigh-Taylor and Kelvin-Helmholtz hydrodynamic instabilities occurring on the jet-fluid interface. This effect is described experimentally in A. Arensburg, S. Wald and S. Goldsmith, J. Appl. Phys. 73 (5) (1993), incorporated by reference.
The outcome of this effect is that a major part of the radiation directly excites specific chemical bonds within the treated materials as compared to the global excitation in the CPI case. The photon wavelength distribution is similar to that of a black body. For a typical PPI plasma most of the photons are in the range of 100-200 nm. This is in the range most relevant for the cracking of many important chemical bonds, such as halogen-carbon bonds in halo-organic compounds. Therefore, the effect of the plasma jet is more similar to photolysis processes usually carried out by UV lamps or by high intensity lasers. Laser (and UV) decomposition is well known, but it is difficult and expensive to produce a laser pulse of several milliseconds in the power range of 100 MW, which is, however, easily obtained by PPI. Furthermore, there is no plasma jet effect leading to an increase in RHT.
The incineration of chemical wastes by using pulsed plasma has been previously described in a number of publications.
U.S. Pat. No. 3,494,974 describes the pyrolysis of 1,2-dichlorotetrafluoroethane in a plasma jet to produce trifluromethane. The plasma torch in this patent is a conventional one: low-density, low power and not produced by a confined discharge. The pulsing mode is used in this patent only as a result of the sequential character of the process for the production of trifluromethane. It is therefore in truth more comparable to a CPI process.
In a recent publication by H. Kohno et.al. "Destruction of Volatile Organic Compounds Used in the Semiconductor Industry by a Capillary Tube Discharge Reactor", IEEE Trans. on Ind. Appl. (1995) pp. 1445-1452, confined discharge is used for plasma production. However, the discharge is done in a gaseous environment without the presence of any ablating material. No ablation mechanism occurs, the plasma density is low and no jet-flow momentum effects take place.